Using KDbgCtrl

The KDbgCtrl (Kernel Debugging Control, kdbgctrl.exe) tool can be used to control the kernel debugging connection from the target computer.

To use this tool, your target computer must be running Windows Server 2003 or a later version of Windows.

KDbgCtrl can control five different settings: Full Kernel Debugging, Automatic Kernel Debugging, User-Mode Error Handling, Blocking of Kernel Debugging, and the size of the DbgPrint buffer.

To use KDbgCtrl, you must have already enabled kernel debugging in the boot settings of the target computer before the last boot. KDbgCtrl cannot be used to enable kernel debugging if this was not done. See Boot Parameters to Enable Debugging for details on these boot settings.

Full Kernel Debugging

When Full Kernel Debugging is enabled, a kernel debugger running on the host computer can break into the target computer. The target computer will break into the kernel debugger if a kernel-mode exception is hit. Messages from the target to the host, such as DbgPrint output, symbol load messages, and redirected user-mode debuggers, are also allowed.

If this setting is disabled, all signals from the host computer will be ignored by the target.

Full Kernel Debugging is enabled by default. To check the current setting value, use kdbgctrl -c. To disable this setting, use kdbgctrl -d. To enable this setting, use kdbgctrl -e.

If you wish to check the current setting and use it to control execution within a batch file, you can use the kdbgctrl -cx command. For details on this command, see KDbgCtrl Command-Line Options.

Automatic Kernel Debugging

If Full Kernel Debugging is enabled, the current setting for Automatic Kernel Debugging is immaterial -- all communication is permitted.

When Full Kernel Debugging is disabled and Automatic Kernel Debugging is enabled, only the target computer can initiate a debugging connection.

In this case, only a kernel-mode exception, breakpoint, or other kernel-mode event will cause a connection to be established. The connection will not be established for DbgPrint output, symbol load messages, redirected user-mode debugger input and output, or other similar messages -- these will be stored in the DbgPrint buffer instead of being sent to the kernel debugger.

If an exception or event causes the target to break into the kernel debugger, then Full Kernel Debugging will be automatically turned on, just as if you had executed kdbgctrl -e.

Automatic Kernel Debugging is disabled by default (although this is immaterial unless Full Kernel Debugging is disabled as well). To check the current setting value, use kdbgctrl -ca. To disable this setting, use kdbgctrl -da. To enable this setting, use kdbgctrl -ea.

User-Mode Error Handling

When User-Mode Error Handling is enabled, some user-mode events will cause the target computer to break into the kernel debugger.

Specifically, all int 3 interrupts -- such as breakpoints inserted into the code by a debugger or calls to DbgBreakPoint -- will cause a break into the kernel debugger. However, standard exceptions -- such as access violations and division by zero -- will usually not be sent to the kernel debugger.

If a user-mode debugger is already attached to the process, this debugger will capture all user-mode errors, and the kernel debugger will not be alterted. For the precedence ranking of the various user-mode error handlers, see Enabling Postmortem Debugging.

For User-Mode Error Handling to function, either Full Kernel Debugging or Automatic Kernel Debugging must be enabled as well.

User-Mode Error Handling is enabled by default. To check the current setting value, use kdbgctrl -cu. To disable this setting, use kdbgctrl -du. To enable this setting, use kdbgctrl -eu.

Blocking Kernel Debugging

In some cases you might want to set up the target computer for kernel debugging, but wait to enable kernel debugging until after the target computer is started. You can do that by blocking kernel debugging.

To block kernel debugging, set up the target computer by using commands similar to the following:

When you restart the target computer, it will be prepared for kernel debugging, but kernel debugging and User-Mode Error Handling will be disabled. At that point, a host computer will not be able to attach to the target computer, bug checks will not be caught by the kernel debugger, and user-mode exceptions will not cause a break in to the kernel debugger.

When you are ready, you can enable kernel debugging (without restarting the target computer) by entering the following commands.

You can use kdbgctrl -cb to check whether kernel debugging is blocked.

The DbgPrint Buffer Size

The DbgPrint buffer stores messages that the target computer has sent to the kernel debugger.

If Full Kernel Debugging is enabled, these messages will automatically appear in the kernel debugger. But if this option is disabled, these messages will be stored in the buffer. At a later point in time, you can enable kernel debugging, connect to a kernel debugger, and use the !dbgprint extension to see the contents of this buffer. For more information about this buffer, see The DbgPrint Buffer.

The default size of the DbgPrint buffer is 4 KB on a free build of Windows, and 32 KB on a checked build of Windows. To determine the current buffer size, use kdbgctrl -cdb. To change the buffer size, use kdbgctrl -sdbSize, where Size specifies the new buffer size. For syntax details, see KDbgCtrl Command-Line Options.

If you are disabling all kernel debugging, you may also wish to increase the size of the DbgPrint buffer. This insures that all messages will be saved in case you need to see them later. If you have a megabyte of memory to spare, you might use the following command: